1. Field of the Invention
The present invention relates to a PIN photodiode used for the reception of light signals in an optical communication field.
2. Description of the Related Art
In general, a PIN photodiode comprises a P-type semiconductor, a N-type semiconductor, and an intrinsic semiconductor layer interposed there-between. The PIN photodiode is typically used as a light-receiving element for converting an optical signal into an electric signal as it possessesexcellent characteristics in converting incident photons into electrons, mainly due to its ability to increase the width of a depletion layer of the PIN photodiode.
FIG. 1a is a top plan view of a conventional PIN photodiode, and FIG. 1b is a cross-sectional view taken along the line A-B of the PIN photodiode shown in FIG. 1a. 
Referring to FIGS. 1a and 1b, the conventional PIN photodiode comprises: a non-doped InGaAs layer 2, a P− InP layer 3, a donut-shaped SiN insulation layer 4 with a circular window formed in the center of the insulation layer, a P+ bonding layer 5 formed on the InP layer 2 within the circular window, an electrode pad 6 formed on the SiN insulation layer 4 in contact with a part of the P+ bonding layer 5, and an anti-reflection layer 7 formed on the P+ bonding layer 5 in which the electrode pad 6 is not in contact, wherein the above layers and the electrode pad are formed on a N+ InP substrate in this order. The PIN photodiode operates when a negative electric field is applied to the P+ bonding layer 5 so that the P+ bonding layer 5 is sufficiently depleted.
In the above PIN diode, if a negative electric field is applied to the P+ bonding layer through the electrode pad, the bonding is laterally extended as compared to the case in which no electric field is applied, so that the area of the P+ bonding layer increases, thus causing a parasitic effect that increases the electrostatic capacitance of the element. However, as the electrostatic capacitance increases the RC time constant also increases, thereby deteriorating the high-speed operating property of the diode element.
Therefore, there is a need for an improved PIN photodiode that overcomes the above-described problem.